Unexpected "color" transition in half transparent cylinder with Cycles
It seems that the reason of this shading 'glitch' is topology of your Cylinder. I don't know exactly why this is happening, maybe because of stretched faces (or for sure). When you scale it down on the X axis (to make it shorter) problem disappears.



You have two options:


Add Subdivision Surface modifier.





Add Loop Cut in the middle of the Cylinder (or couple of them to be sure that it doesn't happen again.




Personally I would go with second one as this is 'lighter' solution.